1. Field of the Invention
The present invention concerns a method of forming an electrophotographic photoreceptor and a method of drying a coating film.
2. Description of the Related Art
An electrophotographic photoreceptor for use in image forming apparatus such as copying machines, printers and facsimile units (hereinafter simply referred to as a photoreceptor) is formed by coating an organic photosensitive layer to the outer circumferential surface of a hollow cylindrical conductive substrate. Development has been made to electrophotographic photoreceptors for coping with a demand for higher performance and most of them have a laminate structure formed by laminating an undercoat layer, a charge generating layer, a charge transporting layer, a protective layer, etc. In the invention, a layer including the undercoat layer, the charge generating layer, the charge transporting layer, and the protective layer is generally referred to as a photosensitive layer. Since the undercoat layer and the protective layer are disposed for improving the performance of the electrophotographic photoreceptor and not always necessary, a layer comprising two layers of a charge generating and a charge transporting layer, or a layer in which the charge generating layer and the charge transporting layer are constituted as a single layer is also referred to as the photosensitive layer.
As the image forming method using the electrophotographic photoreceptor, the following electrophotographic image forming method utilizing the photoconductive phenomenon of the photoreceptor is used generally. At first, the photoreceptor is placed in a dark place and, after uniformly charging the surface of the photoreceptor by charging means, exposure corresponding to image information is applied to selectively discharge the surface charges in the exposed area. This results in a state where surface charges are remained only in the not-exposed area of the photoreceptor and difference is caused between the amount of surface charges in the exposed area and the amount of surface charges in the not-exposed area, to form electrostatic latent images. Then, fine colored charged particles referred to as a toner are deposited to the formed electrostatic latent images, for example, by electrostatic attraction to form toner images as visible images. The formed toner images are transferred optionally on a transfer material such as paper and then fixed to form images.
The photoreceptor used in the image forming apparatus of forming images by way of the series of electrophotographic processes described above is required, as basic characteristics, that it is excellent in electric characteristics, for example, it is excellent in the charge retainability, causes less discharges in the dark place, is excellent in the photosensitivity and discharges static charges rapidly by irradiation of light. Further, it is also required for the photoreceptor that the electric characteristics described above are stable even after repetitive use so that uniform images can be formed for a long period of time (repetitive stability), the electric characteristics are stable so that uniform images can be formed irrespective of the change of temperature and humidity (circumstantial stability), etc. In order to improve the repetitive stability and the circumstantial stability, it is necessary to improve the durability against electrical and mechanical external forces.
Durability against the electrical and mechanical external force includes, for example, durability against degradation of a surface layer caused by deposition of active substances such as ozone and NOx (nitrogen oxide) generated by corona discharge during charging, and wear resistance against wear and damages caused by the transfer material such as paper upon transfer and they are determined, particularly, depending on the surface state of the uppermost surface layer. For example, in a case where the surface state is not smooth but includes unevenness, frictional force during transfer increases in the portion of the surface layer to result in a problem such as occurrence of surface damages to lower the durability against the mechanical external force. Further, in a case where voids are present in the surface layer, for example, a surface area on which the active substances generated are deposited during charging increases to deteriorate the durability against the external electric force. Accordingly, it is indispensable to smooth the uppermost surface layer of the photoreceptor in order to improve the repetitive stability and the circumstantial stability.
An electrophotographic photoreceptor is manufactured by way of a coating step of coating a coating solution in which ingredients of a photosensitive layer such as an organic functional material, a binder resin, etc. is dissolved or dispersed in a solvent medium by way of a spraying method, wring coating method, roll coating method, blade method or dipping method on a conductive substrate at a uniform thickness (the coating solution coated on the conductive substrate is hereinafter referred to as a coating film), and a drying step of drying the coating film thereby removing the solvent contained in the coating film. Existent drying step has been conducted so far by a batch system of loading a predetermined number of conductive substrates each formed with a coating film in an in oven that blows a hot blow at a temperature higher than the boiling point of the solvent medium and drying them or a continuous system of passing an electroconductive substrate formed with a coating film in a heat treating furnace in which a plurality of heaters are disposed.
However, in a case of drying the coating film by the hot blow or the heater and the like as described above, the surface of the coating film results in a portion where the heat from the hot blow or the heater is directly applied and a portion where the heat is applied indirectly to cause unevenness in the heating to the surface of the coating film to result in unevenness also on the surface state of the coating film after drying. Further, the surface of the coating film is dried prior to the inside of the coating film and the surface forms an extremely dense hardened film by the hardening under drying. In the hardened film, a gas of the solvent inside the coating film which is evaporated by heating is difficult to escape from the surface of the coating film to sometimes result in drawbacks such as occurrence of bubbles or pinholes at the surface and in the inside of the coating film or flicking of the coating film tending to peel the coating film coated on the upper layer to the coating film surface. Since, such defects, if any, on the surface layer of the photoreceptor tend to cause cracking or peeling to deteriorate the durability against the external electrical and the mechanical force, favorable repetitive stability and circumstantial stability can not be obtained.
Further, since the hardened film makes the solvent gas inside the coating film less escaping from the surface of the coating film, this makes the drying time extremely long for the inside of the coating film. For example, it takes from one to several hours for drying the coating film having the hardened film by the hot blow, heater or the like. When it takes a long time for drying as described above, a large-scale continuous drying furnace or batch type oven is necessary in the production line in order to produce the photoreceptors in a great amount, as well as it takes a much cost for the operation and the control thereof. On the contrary, in a case where the drying time is shortened in order to solve such problems, the solvent medium remains inside the coating film to deteriorate the electric characteristics.
As a method of solving such problems, there have been proposed a method of using an far infrared heater and drying the coating film under heating by absorption of infrared rays to constituent materials per se of the coating film (refer, for example, to Japanese Unexamined Patent Publication JP-A 3-233885(1991), Japanese Examined Patent Publication JP-B2 5-50742(1993), and Japanese Unexamined Patent Publication JP-A 11-311871(1999)), a method of using radio frequency or induction heating and drying the coating film under heating by vibrating the molecules in the constituent material of the coating film (refer, for example, to Japanese Unexamined Patent Publication JP-A 58-102238(1983)), a method of heating a metal conductive substrate by induction heating and drying the film by the heat generation from the conductive substrate (refer, for example, to Japanese Unexamined Patent Publication JP-A 2003-275670), etc.
Since the drying methods for the coating film disclosed in JP-A 3-233885, JP-B2 5-50742, JP-A 11-311871, JP-A 58-102238 and JP-A 2003-275670 are methods of directly heating the coating material itself or indirectly heating to dry the inside of the coating film by heating the conductive substrate, it is described that the unevenness of heating can be decreased. Further, it is described that since the coating film can be dried from the inside and the hardened film is less formed to the surface of the coating film by such methods, the inside of the coating film can also be dried by removing the solvent medium therefrom and the drying time can be shortened.
However, in the drying methods described above, the heat efficiency is extremely higher compared with the heating method by the hot blow, heater or the like and the temperature of the coating film is sometimes increased abruptly, making it difficult to control the temperature of the coating film. In a case where the temperature of the coating film increases abruptly, this result in problems that the temperature of the coating film increases to higher than the heat resistant temperature of the photoreceptor to deteriorate the electric characteristics of the photoreceptor the coating film is heated to a temperature exceeding the boiling point of the solvent medium to generate a great amount of bubbles. Further, since the object of the direct heating in the heating method described above is a coating film or a conductive substrate, the temperature in the atmosphere at the periphery of the coating film rises only by the temperature rise of the coating film or the conductive substrate. Accordingly, when the temperature of the coating film rises abruptly described above, the temperature rise of the atmosphere can not follow the temperature rise of the coating film and the difference of the temperature between the atmosphere and the coating film becomes excessive. The gas of the solvent evaporated inside the coating film stagnates near the surface of the coating film in the process where the gas tends to escape from the surface of the coating film. In a case where the temperature difference increases between the coating film and the atmosphere, the stagnating gas of the solvent medium may sometimes be liquefied again upon escape from the surface of the coating film to the atmosphere thereby sometimes causing unevenness at the surface of the coating film. Further, an additional time is further required for drying the re-liquefied solvent gas, this also results in a problem of extending the drying time.